In general, a mobile communication system is configured to monitor a synchronization state between a radio base station and a mobile station. For example, in a mobile communication system of the WCDMA scheme, the following two indexes are defined to determine a downlink synchronization state in a physical layer.                Dedicated physical control channel (DPCCH) quality        Cyclic redundancy check (CRC) check result        
In this case, the DPCCH quality corresponds to reception quality of pilot symbols or TPC (transmission power control) bits, for example, an SIR (signal-to-interference power ratio) or a reception level, and the CRC check result corresponds to a block error rate.
A determination interval for determining the downlink synchronization state in the physical layer, for example, is 160 ms. As described below, the determination for the radio link failure (RLF) state in the RRC layer is performed based on the determination result of the downlink synchronization state of every 10 ms.
That is, in the mobile communication system of the WCDMA scheme, the determination for the radio link failure state in the RRC layer based on the downlink synchronization state reported from the above-described physical layer is defined, and the operation of the mobile station UE in the radio link failure state is also defined.
The determination for the radio link failure state in the RRC layer described above and the operation of the mobile station UE in the radio link failure state will be described with reference to FIG. 14.
In step S102, when continuously receiving “Out-of-sync” from a physical layer N313 times, that is, a report of “synchronization state: NG” (S102: YES), the mobile station UE activates a timer T313 (S104). Steps S102 and S104 may be performed only when the timer T313 is not activated.
Meanwhile, in step S106, when continuously receiving “In-sync” from the physical layer N315 times, that is, a report of “synchronization state: OK” (S106: YES), the mobile station UE stops the timer T313 (S108). Steps S106 and S108 may be performed only when the timer T313 is activated.
When the T313 expires (step S110: YES), the present operation proceeds to step S112. However, when T313 has not expired (step S110: NO), the present operation returns to step S102. Step S110 may be performed both when the T313 is activated and when the T313 is not activated.
The processes of steps S102 to S110 may be always performed when a radio base station eNB and the mobile station UE are in a connection state. Alternatively, the processes of steps S102 and S110 may be performed when the radio base station eNB and the mobile station UE are in the connection state and at every 10 ms. Here, 10 ms indicates one radio frame. In addition, the connection state may be an RRC_Connected state.
In step S112, the mobile station UE regards the communication state between the radio base station eNB and the mobile station UE as the radio link failure state.
The radio link failure state may be called an RLF (Radio Link Failure) state, and refers to a state in which a communication quality between the radio base station eNB and the mobile station UE considerably deteriorates and thus communication is not possible.
In step S114, the mobile station UE clears the communication configuration between the radio base station eNB and the mobile station UE.
In step S116, the mobile station UE performs reconnection process. Here, the reconnection process may be called “Cell Update process”. In addition, the mobile station UE may perform the reconnection process, when there is no process being performed or when the operation in the radio link failure state is not defined during a process being performed.
In such a case, the mobile station UE performs the operation in the radio link failure state during the process being performed, when there is process being performed and when the operation in the radio link failure state is defined during the process being performed.
The above-described parameters such as T313, T315 and N313 correspond to hysteresis or the number of protection steps, and are used for determining the radio link failure state with high accuracy and at an appropriate timing.
However, the mobile communication system adopts discontinuous reception (DRX) control, in order to reduce the power consumption of the mobile station UE, that is, for battery saving purpose.
For example, in a mobile communication system of the LTE (Long Term Evolution) scheme which is the next generation of the WCDMA scheme, the discontinuous reception control is applied while the radio base station eNB and the mobile station UE are connected to each other and when there is no data to be communicated, and the mobile station UE in the discontinuous reception state is configured to receive a downlink control signal transmitted via a physical downlink control channel (PDCCH) periodically, that is, discontinuously.
In such a case, since the mobile station UE has only to receive the downlink control signal transmitted via the physical downlink control channel PDCCH discontinuously, instead of all timings, the power consumption of the battery can be reduced.
In more detail, as shown in FIG. 15, the mobile station UE is configured to receive the downlink control signal transmitted via the physical downlink control channel PDCCH, and other transceivers are configured to be turned off, only at the reception interval (5 ms in the example of FIG. 15) set in each DRX cycle (1280 ms in the example of FIG. 15). As a consequence, the power consumption of the battery of the mobile station UE can be reduced. The reception interval may be called On-duration.